1. Field of the Invention
This invention relates to a method for manufacturing a glass fiber partially or entirely made of chalcogenide glass or oxychalcogenide glass and, more particularly, to a method for manufacturing a glass fiber useful for an optical fiber containing, in a core of the optical fiber, light emitting substances for optical amplification fiber.
2. Description of Related Art
An inexpensive, high efficiency, 1.3-micron-meter band optical amplifier has lately been expected in the art of optical telecommunication. The currently used 1.3-micron-meter band optical amplification medium is an optical fiber containing Pr.sup.3+ ions as light emitting substances in the core of the fiber. Chalcogenide glass has been used as a host glass to which the Pr.sup.3+ ions are added. An amplifier with very high efficiency can be fabricated by using the chalcogenide glass.
To obtain an amplifier with higher efficiency, it is desirable to dope more light emitting substances in the core. The chalcogenide glass, however, generally does not solve enough ionic substances serving as the light emitting substances. From this viewpoint, we have paid attentions, as a possible host glass capable of solving more ionic substances, some sulfuric glasses in which sulfur is mainly used as the chalcogen element.
Meanwhile, to use such a glass as an optical amplification medium, the glass is required to be formed into a single mode optical fiber form. Japanese Unexamined Patent Publication (KOKAI) Showa No. 64-3,031 discloses such a method for forming the chalcogenide glass into a fiber. This method is based on a process using a crucible.
Because the method disclosed in Japanese Unexamined Patent Publication Showa No. 64-3,031 is the process using a crucible, a fiber having a core with a diameter of around 15 to 16 micron meters or less, which is particular to a single mode fiber, is hardly fabricated. A general method to fabricate a single mode fiber is to form a preform with a large ratio of a clad diameter to a core diameter by a method of a rod-in-tube, extrusion, etc., and then to draw the preform into a fiber in applying heat to a part of the preform to soften the preform.
The chalcogenide glass, however, is less stable, thereby rendering actually impossible formation of an optical fiber in use of the conventional preform method as it is. Under this circumstance, we have tried some single mode fiber fabrications from a sulfuric chalcogenide capable of containing more light emitting substances in use of the preform method.
The preform method, however, volatilizes the chalcogen element such as sulfur from a side face of a preform rod or jacketing tube that is directly exposed to the gas phase atmosphere during drawing. The sulfuric chalcogenide particularly experienced remarkable sulfuric volatilization. If the chalcogen element such as sulfur volatilizes, a surface composition of the preform rod or jacketing tube may shift from an inner composition. Sulfuric metallic chalcogenide glass has a narrower range for forming glass than arsenic-sulfuric glass and is less stable against crystallization. If chalcogen element such as sulfur excessively volatilizes from the surface of the preform rod or jacketing tube, this volatilization induces composition shifts or metamorphosis on the surface, thereby likely causing surface crystallization. In fact, the fiber lost the fiber's strength due to crystal depositions. In some occasions, the glass cannot be even drawn due to significant surface crystallization.